The following paragraphs contain some discussion, which is illuminated by the innovations disclosed in this application, and any discussion of actual or proposed or possible approaches in this Background section does not imply that those approaches are prior art.
Natural resources such as oil and gas residing in a subterranean formation or zone are usually recovered by forming wells through the formation. The formation of a well first involves drilling a wellbore down to the subterranean formation while circulating a drilling fluid in the wellbore. The drilling fluid is usually circulated downwardly through the interior of a drill pipe and upwardly through the annulus, which is located between the exterior of the pipe and the walls of the wellbore. After terminating the circulation of the drilling fluid, a string of pipe, e.g., casing, is run in the wellbore. Next, primary cementing is typically performed whereby a cement slurry is placed in the annulus and permitted to set into a hard mass (i.e., sheath) to thereby attach the string of pipe to the walls of the wellbore and seal the annulus. Subsequent secondary cementing operations may also be performed. One example of a secondary cementing operation is squeeze cementing whereby a cement slurry is employed to plug and seal off undesirable flow passages in the cement sheath and/or the casing.
One problem commonly encountered while drilling and cementing the wellbore is the presence of one or more permeable zones in the subterranean formation. The permeable zones may be, for example, vugs, voids, naturally occurring fractures, or induced fractures that occur when weak zones have fracture gradients exceeded by the hydrostatic pressure of the drilling fluid or the cement slurry. In some cases, weak zones may contain pre-existing fractures that expand under the hydrostatic pressure of the drilling fluid or the cement slurry. During the drilling operation, such permeable zones may result in the loss of the drilling fluid circulation in the wellbore such that the drilling operation must be delayed for technical or commercial reasons. Further, during the cementing operation, at least a portion of the cement slurry may be lost to the subterranean formation due to the presence of such permeable zones. Consequently, the cement slurry may fail to completely fill the annulus from top to bottom. Dehydration of the cement slurry also may occur, compromising the strength of the cement that forms in the annulus.
Traditional methods of overcoming the above described problem include sealing the permeable zones using thixotropic cements, bentonite diesel oil, bentonite cement diesel oil, and sodium silicate solutions in combination with calcium salt sweeps. However, these materials have their own share of limitations. For example, they may lack sufficient compressive strength to adequately plug the permeable zones, particularly if they become contaminated with the drilling fluid. Further, they often develop compressive strength very slowly, extending the period of time required to complete the formation of the wellbore. Well fluids containing flaky materials, e.g., mica and shredded cellophane, or granular materials, e.g., ground nutshells and carbonates, have also been developed to seal permeable zones. However, the flaky or granular materials may be washed away from highly permeable zones by the fluids being circulated through the wellbore. The foregoing plugging materials also may be difficult to remove from the permeable zones when hydrocarbon production is required, resulting in damage to the formation. A need therefore exists to develop improved materials for preventing the lost circulation of fluids during the formation of wells in subterranean formations.